Basic theory of pneumatic conveying Principles of air flow conveying in vertical pipelines
Suppose the airflow velocity of pure air in the pipeline is v. When transporting fibers in the pipeline, the void ratio of the fiber medium is ε, then the flow rate permeating through the voids of the fiber medium is v/ε. When the flow velocity v is small, the fiber does not When v gradually increases, the entire fiber medium expands and increases ε, and the fiber begins to rise. However, due to the increase of ε in the upper part, the v/ε value decreases, so the fiber falls again. This phenomenon of rising and falling It is called fluidization, but the fibers remain in the pipe and cannot achieve the purpose of transportation. When v continues to increase and exceeds the terminal velocity vT of the fiber medium, it will no longer fall after rising and continue to rest, forming a transportation process.
When assuming pneumatic transportation, the leisure speed of the fiber in the pipe is vf, which is generally lower than the air flow speed v, and the relative speed between the two is v-vf , where During the normal and stable transportation process of vertical pipelines, v-vf =vT, that is, v=vT +vf . Because vf =v-vT, when the fiber medium is suspended in the vertical pipe, vf =0, which makes the fiber medium in The airflow velocity that produces suspension in vertical pipes is called suspension velocity. It can be seen that the suspension speed is numerically equal to the settling velocity (terminal velocity), that is, when the air flow velocity reaches the free settling terminal velocity (terminal velocity), the fiber medium is suspended in the vertical pipe. When the air flow velocity is greater than the terminal velocity of the fiber medium, Only when the final speed vT can normal transportation of vertical pipelines be carried out. The airflow speed is generally controlled at 1.5 to 2.0 times the terminal speed.
When transporting fiber media in vertical pipelines, if the flow rate is reduced to a certain extent, there will be more fiber media in some sections and less fiber media in some sections. In a section with more fibers, ε is smaller. The flow rate v/ε of air passing through the gaps in the fiber medium is relatively high, causing surging phenomena, resulting in uneven flow. If the flow rate decreases further, blockage will occur. Whenever a spiral break occurs in a straight pipeline, due to the increase in pipe wall friction and the energy consumption of the spiral break, blockage will easily occur, requiring an increase in the conveying speed.
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